Ethernet-based products for factory automation and related fields are widely used for communicating between data processing systems and peripheral devices. Local area networks (LANs) interconnect factory equipment and other devices such as programmable logic controllers (PLCs), bridges (e.g., the BP85 Bridge Plus and BM85 Bridge Multiplexer by Schneider Automation, Inc.), industrial power monitors (e.g., the CM 4000 by Schneider Electric N.A.), programmable logic controllers (PLCs) (e.g., any of the Quantum PLCs by Schneider Automatoin, Inc.), and computer work stations for monitoring and programming PLCs and other devices related to factory automation The MODBUS protocol is widely used for factory automation applications. The MODBUS protocol is described in the “MODBUS Protocol Reference Guide,” publication PI-MBUS-300 by Schneider Automation, Inc., which is incorporated herein by reference. MODBUS Plus is a LAN protocol for industrial control applications. Applications of the MODBUS Plus protocol are described in the “MODBUS Plus Network Planning and Installation Guide,” 890 USE 100 00 Version 3.0, Schneider Electric, April 1996, which is incorporated herein by reference.
The MODBUS protocol is well known and is described, for example, on the World Wide Web (Web) at http://www.modicon.com/techpubs/toc7.html, which is incorporated herein by reference along with all related Web pages.
The emergence of Ethernet as the de facto standard of corporate enterprise systems has led to the use of Ethernet in factory networking. This, in turn, has led to the development of MODBUS/TCP which combines Ethernet, TCP/IP (transmission control protocol/internet protocol), and MODBUS messaging to achieve scalable networks for factory automation equipment, using these well-known protocols and specifications. The MODBUS/TCP protocol is described in Swales, “Open MODBUS/TCP Specification,” Release 1.0, Mar. 29, 1999, which is incorporated herein by reference. Sample source code for a MODBUS/TCP driver and Swales, “Open MODBUS/TCP Specification” is available on the Web at http://www.modicon.com/openmbus, which is incorporated herein by reference along with all related Web pages. The MODBUS protocol is described on the Web at http://www.modicon.com/techpubs/intr7.html, which is incorporated herein by reference along with all related Web pages. Different networking schemes relating to factory automation are described in U.S. Pat. Nos. 6,151,625; 5,805,442; 5,251,302; and 5,699,350, which are incorporated herein by reference.
A MODBUS frame includes an address field for storing a device identifier (ID). The device ID identifies the slave device to which the MODBUS frame is to be sent when the message is being sent from a master device. When the frame originates at a slave device and is to be sent to a master device, the device ID identifies the slave device from which the MODBUS frame is sent. Thus, a master addresses a slave by placing the slave address in the address field of the message, and when the slave sends its response, it places its own address in the address field to let the master know which slave is responding.
Although MODBUS/TCP has facilitated internetworking of factory automation devices, there are significant restraints on the addressing scheme used by MODBUS/TCP. Presently, both ends of a MODBUS/TCP connection are manually configured such that every MODBUS master device has a look-up table of destination IP addresses for every possible MODBUS device ID value. In many cases, the look-up tables in the MODBUS master devices are identical, and thus, redundant. The tables require updating by an engineer or skilled labor. Devices may change address without notice. The tables become outdated and the operator receives an error message as notification to update a table—an inefficient and awkward error messaging system. As the industry moves toward interoperatability, that is, seamless data transfer between data transfer protocols the number of third party devices requiring addressing increase, further increasing maintenance and complexity in addressing schemes.